1. Field of the Invention
The present disclosure relates to a substrate on which a monocrystalline ZnO is formed. The substrate can be used for fabricating a light-emitting diode element, a solar cell, and an electric device using semiconductor such as gallium nitride (GaN) or zinc oxide (ZnO).
2. Description of the Related Art
Recently, a semiconductor element composed of a nitride semiconductor such as gallium nitride (GaN) has been researched and developed actively. A semiconductor light-emitting element composed of the nitride semiconductor comprising aluminum nitride (AlN), gallium nitride (GaN) or indium nitride (InN), or mixed crystal thereof emits light in a wide wavelength region from ultraviolet or blue to infrared by varying its film composition. A visible-range light-emitting diode using the nitride semiconductor has already been commercially-available. In order to suppress the carrier recombination due to non-luminescence transition derived from lattice defects or penetrating cracks, a nitride semiconductor film having significantly low defects in the crystal has to be prepared. This requires a monocrystalline substrate such as a sapphire substrate, but such a monocrystalline substrate is very expensive.
Japanese Laid-open patent publication No. 2009-200207 (Hereinafter, Patent Document 1) attempting to solve the above-mentioned problem discloses a method for preparing a polycrystalline nitride semiconductor film on a graphite substrate by a pulse sputtering method. However, since the GaN film obtained in accordance with Patent Document 1 is polycrystalline having many grain boundaries in the crystal, it is not suitable for preparing a light-emitting diode with high performance.
Both of GaN and ZnO have a wurtzite-type crystal structure. The a-axis mismatch factor between GaN and ZnO is 1.8%. The c-axis mismatch factor therebetween is 0.4%. Both of these values are very small.
Accordingly, a ZnO monocrystalline substrate is useful not only as a substrate for homoepitaxial growth to form a ZnO semiconductor layer but also as a substrate for heteroepitaxial growth to form a GaN semiconductor layer. Thus, the light-emitting diode element having the ZnO semiconductor layer or the GaN semiconductor layer formed on the ZnO monocrystalline substrate has been proposed.
The ZnO monocrystalline substrate necessary to form a high quality semiconductor such as GaN or ZnO is, however, very expensive. Furthermore, it is difficult to fabricate a ZnO monocrystalline substrate with large area.
As method for forming a ZnO substrate, a vacuum film-forming method such as a sputtering method, a reactive plasma deposition method, a metal organic chemical vapor deposition (MOCVD) method, a pulse laser deposition method, or a molecular beam epitaxy method has been utilized. In order to form a monocrystalline ZnO with a small amount of defects by the vacuum film-forming method, it is necessary to introduce oxygen and to heat a substrate to approximately 500 to 800 degrees Celsius. In such a high temperature oxidation atmosphere, when a graphite substrate is utilized, the graphite substrate is easy to be deteriorated. Accordingly, it is difficult to prepare the monocrystalline ZnO on the graphite substrate.